Gypsum based panel and method for making gypsum based panel

ABSTRACT

A method of forming a gypsum-based panel product including the steps of: mixing calcined gypsum, water, a foaming agent and a core strengthening agent to form a foamed slurry; depositing the slurry; shaping the slurry to form a board having a core and core surfaces, optionally sandwiched between sheets of liner; and allowing the core to set, and drying the board, wherein the strengthening agent is dextrin, used in an amount not less than 0.1% by weight, based on the amount of calcined gypsum, so that, after drying, there is provided a strengthening dispersion of dextrin throughout the core. Less than 5% dextrin in preferred.

BACKGROUND TO THE INVENTION

1. Field of the Invention

The present invention relates to a gypsum-based panel such asplasterboard, for example, and a method for making such a gypsum-basedpanel.

2. Related Art

Plasterboard is well known for its uses in the construction industry. Itmay be used in interior wall and ceiling construction. Its mainadvantages over more traditional materials are its low manufacturingcost, low installation cost and fire retardance.

Plasterboard typically has a gypsum-based core sandwiched between twosheets of liner paper. However, it is possible to form a similar productusing different liners, or with no liners. For this reason, the moregeneral term “gypsum-based panel” is used herein. It is to be understoodthat plasterboard is one class of gypsum-based panel.

Plasterboard is typically formed in a continuous process in which aslurry is provided by mixing dry ingredients with water and a foamingagent. Mixing techniques for providing the slurry vary by region, butgenerally the slurry is mixed and then continuously deposited betweentwo continuously supplied sheets of liner paper. One of the sheets ofliner paper (typically the face paper) is folded and bonded to the othersheet of liner paper (typically the backing paper). The product of thisprocess is pressed to the required thickness by a forming plate. Theslurry is then allowed to set between the two liner paper sheets. Thecontinuously-produced board is then cut into panels of a predeterminedlength which are then dried in kilns to remove excess water.

Typically, the dry ingredients of the slurry include calcium sulphatehemihydrate (CaSO₄.0.5H₂O), also known as calcined gypsum, acid-modifiedstarch for promoting the bond between the liner paper and the core, andoptional other additives. These are mixed with water and foam or afoaming agent. The purpose of the foam is to include voids in the slurryin order to reduce the density of the set core of the plasterboard.

The calcined gypsum undergoes a hydration reaction in the presence ofwater to form calcium sulphate dihydrate (CaSO₄.2H₂O), or gypsum.Needle-like crystals of gypsum form and it is the resultant network ofthese crystals that lends the finished product the basis for thecompressive strength of the core.

The acid modified (thinned) starch added to the slurry promotes theadhesion of the liner paper to the core. It is believed that the acidmodified starch migrates with water during the board manufacturingprocess to the paper-core interface to promote adhesion, as explained inU.S. Pat. No. 2,207,339, in which an acid modified starch is prepared bymodifying raw starch using dilute nitric acid.

It is also known to apply starches directly to the liner paper, toimprove adhesion of the liner paper to the core. See, for example, U.S.Pat. No. 4,051,291, U.S. Pat. No. 4,117,183, U.S. Pat. No. 4,119,752.

WO 02/12141 discloses the use of starch in plasterboard and explainsthat the starch migrates to the core-paper interface to help the linerpaper bind to the core.

U.S. Pat. No. 3,989,534 suggests that a film former may be added to agypsum slurry in order to improve the stability of a foam introducedinto the slurry to reduce the final density of the product. Suitablefilm formers are cold-water soluble organic compounds such aspregelatinized starch, guar gum or xanthan gum.

FR-A-1412596 discloses the use of dextrins (a form of starch) injointing compounds for use with prefabricated gypsum boards. SimilarlyAU 9221039 discloses the use of a combination of PVA, methyl celluloseand yellow dextrin (see below) in a jointing composition.

U.S. Pat. No. 4,169,747 discloses an accelerator composition forplasterboard, the accelerator comprising finely ground hydrated gypsumand an additive for preserving the gypsum from calcination. The additivemay be lignosulfate, starch, dextrin or sucrose.

U.S. Pat. No. 6,251,979, U.S. Pat. No. 6,340,388, U.S. Pat. No.6,391,958 and U.S. Pat. No. 6,403,688 briefly discuss the use ofstarches or dextrins as an adhesive agent to promote the bonding ofliner paper to the core in plasterboard.

DE-A-3246534 also disclose the use of adhesives in wallboardmanufacture, the adhesive acting to adhere the core to the liner paper,and being one of starch, dextrin or synthetic resin. JP-B-72-051086contains similar disclosure.

WO 97/35888 discloses methods for starch degradation, for use inwallboard.

U.S. Pat. No. 6,319,312 discloses a plasterboard formulation includingan expanded mineral such as perlite in order to reduce the amount ofgypsum required. The formulation requires a combination of syntheticbinders that form a crosslinked network in the plasterboard. Thecombination of synthetic binders is a vinyl acetate emulsion in the formof poly(vinyl acetate) particles in polyvinyl alcohol and water, asource of boron (e.g. borax) and starch. It is suggested that the starchcould be dextrin, although this is not exemplified. The use of perlite(in effect, a lightweight aggregate) in the board required the use ofthe synthetic binders in order to bond the core. Failure to use thesynthetic binders would likely have led to a core with unsuitably lowstrength. It has been found that the formulation exemplified in U.S.Pat. No. 6,319,312 is not effective in providing a suitable plasterboardstructure, for reasons that are not well understood at this time.Furthermore, it is not preferred to use synthetic binders such as PVA inplasterboard made using normal foamed gypsum slurry, since the syntheticbinder is not cost-effective for large-scale industrial plasterboardmanufacture.

WO 99/08978 discloses the addition of trimetaphosphate ions to aplasterboard slurry, to provide an increase in strength of the core. Inaddition, that document discloses that the typical use of anonpregelatinized starch (e.g. an acid modified starch) to promote thepaper-core bond may not be sufficient to prevent weakening of that bondwhen the board becomes wet. Thus, as well as the use of trimetaphosphateion, WO 99/08978 discloses the addition of pregelatinized starch to theslurry. The result of this is that the weakening of the paper-core bondis reduced. The mechanism for this beneficial effect is not explained inWO 99/08978, but it is observed that the pregelatinized starch becomesdistributed throughout the core. Raw starch is pregelatinized by cookingin water at temperatures of at least 185° F. (about 85° C.). Thepregelatinized starch is included in the slurry in an amount between0.08-0.5% by weight, based on the weight of gypsum.

U.S. Pat. No. 5,922,447 discloses the uses of a partially cooked starchin wallboard. the wallboard core contained both gypsum and perlite. Thestarch binds the gypsum particles and perlite spheres and does not tendto migrate to the paper-core interface. The starch is partially cookedin water at temperatures between 150° F. and 185° F. (between about 65°C. and about 85° C.). The starch used is pearl starch (in this case acombination of starch made from corn, potato and/or wheat stock),although acid-modified starches are also suggested.

One measure of the core strength of plasterboard is nail pullresistance. Methods for testing the nail pull resistance of plasterboardare set out in ASTM C473-03 (Standard Test Methods for Physical Testingof Gypsum Panel Products). Ideally, for many applications, plasterboardshould have a low density (usually referred to by the weight per areafor a particular board thickness, e.g. pounds per thousand square feet(lb/MSF)) and a high nail pull resistance.

U.S. Pat. No. 6,783,587 discloses a plasterboard in which starch isadded in an amount between 1.5-3.0% by weight, based on the amount ofgypsum, in order to increase the nail pull resistance of the board. Thestarch used is acid modified starch, of the type typically used topromote binding of the liner paper to the core.

US-A-2003/0084980 discloses a plasterboard formulation includingacid-modified starch and a starch crosslinking agent (e.g. type Nhydrated lime). The cross-linking agent is believed to lock some of thestarch in the core, the remainder migrating to the paper-core interfaceto promote bonding.

US-A-2003/0092784 discloses a polymer-reinforced gypsum material, thepolymer being crosslinked in situ. The polymer used is a syntheticpolymer such as PVP (poly(vinyl pyrrolidone)) and the crosslinkingcomponent is PSS (poly(sodium 4-styrene sulfonate)).

U.S. Pat. No. 5,879,825 discloses the addition of an acrylic polymercomposition, having a glass transition temperature of 15° C. or greater,to plasterboard in order to increase the core strength of theplasterboard.

US-A-2004/0092625, US-A-2004/0092624 and U.S. Pat. No. 6,841,232disclose a gypsum based composite structure such as plasterboard with acellulose ether additive (other than carboxymethylecellulose (CMC)), inorder to improve nail pull resistance and flexural strength.

US-A-2005/0126437 discloses a wallboard core composition including asubstituted starch. The properties of the starch (degree ofsubstitution, degree of polymerization and viscosity) are selected sothat the starch does not dissolve in water at the slurry mixingtemperature, but does dissolve at higher temperatures. In this way,excessive migration of the starch to the core-paper interface isavoided.

SUMMARY OF THE INVENTION

Of paramount importance in the industrial production of gypsum-basedpanel products is the cost of the starting materials, as well as theefficiency of the manufacturing process. Due to the very high volumenature of gypsum-based panel products manufacture, even a smallefficiency saving or a change in starting materials can result in verysignificant overall savings in production costs.

The present inventors have realised that it is possible to utilizedextrin as a core strength-enhancing agent in the manufacture ofgypsum-based panel products, with the unexpected result that the nailpull resistance and/or other strength-related properties of thegypsum-based panel products can be improved. Additionally oralternatively, the panel can be made more lightweight, whilst providingthe same or improved nail pull resistance and/or other strength-relatedproperties of the panel.

In a first aspect, the present invention provides a method of forming agypsum-based panel product including the steps of:

-   -   mixing calcined gypsum, water, a foaming agent and a core        strengthening agent to form a foamed slurry depositing the        slurry    -   shaping the slurry to form a panel having a core and core        surfaces, optionally sandwiched between sheets of liner    -   allowing the core to set, and drying the panel        wherein the strengthening agent is dextrin, used in an amount        not less than 0.1% by weight, based on the amount of calcined        gypsum, so that, after drying, there is provided a strengthening        dispersion of dextrin throughout the core.

Preferably, the dextrin used as the core strengthening agent is formedby thermal degradation.

Preferably, the strengthening agent is used in an amount not less than0.3% by weight, based on the amount of calcined gypsum, preferably notless than 0.4%, 0.5% or 0.6% by weight, based on the amount of gypsum.There may be used 5% or less, or more preferably 3% or less, 2% or less,1.5% or less, 1.2% or less or 1.0% or less core strengthening agent, byweight based on the amount of calcined gypsum.

Preferably, the resultant gypsum-based panel product has a nail pullresistance at least 2% (more preferably at least 4% or most preferablyat least 6%) greater than that of a panel formed by the same method butwithout the strengthening agent. It is considered, without being limitedto this consideration, that the inclusion of dextrin in the core of thefinished product provides additional strength to the core by theformation of a percolating film of dextrin through the core.

Preferably, the resultant panel has a core compressive strength not lessthan that of a panel formed by the same method but without thestrengthening agent.

The moisture content of the dextrin at the time of adding to the slurryis preferably 5% or more. The moisture content of the dextrin at thetime of adding to the slurry may be 12% or less. Most preferably, thedextrin added to the slurry is white dextrin. Alternatively, the dextrinadded to the slurry is yellow dextrin.

The dextrin may be non-pregelatinized. However, preferably the dextrinis pregelatinized.

Preferably, the dextrin is derived from one of corn, wheat, tapioca,potato, rice and sago.

The dextrin may be added to the slurry in the form of granules (e.g. fornon-pregelatinized dextrin) or powder (e.g. for pregelatinized dextrin).

The dextrin may have a composition in which the amount of amylopectin,based on the total amount of dextrin, is more than 70%, preferably about80%. The amount of amylose is preferably less than 30%, preferably lessthan 20%, based on the total amount of dextrin.

Preferably, a liner (most preferably a liner paper) is applied to atleast one of the core surfaces (and preferably a liner paper is alsoapplied to the other core surface) and the resultant plasterboard has aliner-core bond strength not less than that of a plasterboard formed bythe same method but without the strengthening agent.

Preferably, a paper-core bonding agent is added to the slurry, thepaper-core bonding agent being different to the strengthening agent. Inuse, the paper-core bonding agent may migrate towards the paper-coreinterface during processing of the plasterboard to a greater extend thanthe core strengthening agent. However, it has been found that dextrinitself can provide a suitable paper-core bond. Accordingly, thepaper-core bonding agent may be dextrin.

Preferably, the paper-core bonding agent is an acid-modified or oxidisedstarch. The paper-core bonding agent may be added to the slurry in anamount not less than 0.1% by weight, based on the amount of gypsum. Thepaper-core bonding agent may be added to the slurry in an amount notmore than 1% by weight, based on the amount of gypsum. More preferably,the paper-core bonding agent is added to the slurry in an amount notmore than 0.7% by weight (most preferably not more than 0.5% by weight),based on the amount of gypsum.

Preferably, the sum of the amount of paper-core bonding agent and corestrengthening agent is less than 1.5% by weight, based on the amount ofgypsum.

Preferably, the paper-core bonding agent is substantially not soluble inwater at the mixing temperature of the slurry. Preferably, thepaper-core bonding agent is at least partially soluble in water at thedrying temperature of the slurry.

Typically, the panel is produced in a continuous process, the slurrybeing deposited onto a moving conveyor.

In a second aspect, the present invention provides a gypsum-based panelproduct having a core comprising at least partially hydrated calcinedgypsum and a strengthening agent, the core having voids formed therein,the core optionally being sandwiched between sheets of liner, whereinthe strengthening agent is dextrin, used in an amount not less than 0.1%by weight, based on the amount of calcined gypsum, the dextrin beingdistributed in the core so that there is provided a strengtheningdispersion of dextrin throughout the core.

Preferably, the panel comprises not less than 0.3% by weight ofstrengthening agent, based on the amount of calcined gypsum, preferablynot less than 0.4%, 0.5% or 0.6% by weight, based on the amount ofgypsum. The panel may comprise 5% or less, or more preferably 3% orless, 2% or less, 1.5% or less, 1.2% or less or 1.0% or less corestrengthening agent, by weight based on the amount of calcined gypsum.

The panel may have a liner (preferably a liner paper) applied to atleast one of the core surfaces (and preferably a liner is also appliedto the other core surface) to form plasterboard. In this case, theplasterboard may further comprise a paper-core bonding agent, thepaper-core bonding agent being different to the strengthening agent. Thepaper-core bonding agent may be an acid-modified starch. Preferably, thepaper-core bonding agent is present in the plasterboard in an amount notless than 0.1% by weight and not more than 1% by weight, based on theamount of gypsum.

Typically, the sum of the amount of paper-core bonding agent and corestrengthening agent is less than 1.5% by weight, based on the amount ofgypsum.

The panel may be considered to have a notional centre line, equispacedbetween the surfaces of the board. A central region in the panel may bedefined as follows:

-   -   a band extending to two thirds of the thickness of the core and        centred on the centre line.

Peripheral regions of the panel may be defined as follows:

-   -   a first peripheral region of the panel being defined in a band        extending parallel to and incorporating one panel surface and        being 10% of the thickness of the panel;    -   a second peripheral region of the panel, at the opposite side of        the panel from the first peripheral region, said second        peripheral region being defined in a band extending parallel to        and incorporating the other panel surface and being 10% of the        thickness of the panel.

Preferably, of the total amount of dextrin detectable in a cross sectionof the finished product, at least 50% is located in said central regionof the core.

Preferably, at least 50% (and more preferably at least 75%) of the totalamount of paper-core bonding agent present in the board is located inthe first and second peripheral regions of the board.

Preferably, the panel does not include synthetic binder. In particular,preferably the panel does not include polyvinyl acetate particles, e.g.a vinyl acetate emulsion. One reason for this is that synthetic bindersare typically considerable more expensive than polymers derived fromplants or the like. Of course, trace amounts of such materials mayinadvertently occur in the panel, but preferably such trace amounts areless than 0.5% by weight, based on the amount of calcined gypsum andpreferably significantly lower than this (e.g. less than 0.1% byweight).

Furthermore, the panel preferably does not include an expanded mineralfiller material such as perlite. It is preferred instead to generate afoam for the slurry so that voids are formed in the finished panel as aresult of the foam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a bar chart of the average change of compressive strengthof gypsum prismatic blocks containing different starch-basedstrengthening agents, compared to a control sample.

FIG. 2 shows a bar chart of the change in paper-core bond strength ofgypsum prismatic blocks containing different starch-based strengtheningagents, compared to a control sample.

FIG. 3 shows a scatter plot of nail pull resistance of laboratory-madeplasterboard containing different starch-based strengthening agents.

FIG. 4 is a photograph illustrating the difference in response to iodinestaining of plasterboard cores containing different starch-basedstrengthening agents.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As mentioned above, starch can be used in the plasterboard industry toproduce a bond between the two paper liners that the gypsum core issandwiched between. The use of this natural polymer is based on itsgelling, adhesive, film-forming properties and its low cost.

It is known to modify starches by various methods. Modification bydegradation is known. Commercially available degraded starches includeacid-modified starches, oxidized starches and dextrins. These arediscussed below:

Acid modified starches are manufactured by controlled acid-catalysedhydrolysis of the granular starches in aqueous suspension attemperatures below the gelatinisation temperatures, so that the productscan be recovered by filtration, washing and drying. The major reactioninvolved is hydrolytic breakdown, that is, chain scission at glucosidebond. In acid modification 0.2% H₂SO₄ or HCl is used at 50-55° C. for12-14 hours. In an acid-modified starch the acid attacks the starch inthe amorphous areas of the granule more rapidly than the crystallineareas. Initially, the amylopectin in the granule is degraded morerapidly than the amylose. These starches swell a limited amount onheating and the final viscosity reached is a lot lower than thatachieved from a native starch. Typically, the paper-core bonding agentadded to the slurry for plasterboard is an acid modified starch.

The commercial production of oxidized starches involves treating theaqueous starch suspension with sodium hypochlorite solution at pH 8 to10 and at a temperature of 21 to 38° C. The hypochlorite oxidizes alimited number of hydroxyl groups to aldehyde, ketone and carboxylgroups with associated cleavage of the glucoside bonds. The reaction isneutralised when the required level of oxidation has been reached. Theintroduction of carboxyl groups into the linear amylose moleculesreduces the tendency to retrograde. Since carboxyl groups are present,the oxidised starches are anionic at a pH above 5 and therefore can bestained by cationic dyes such as methylene blue. Oxidized starchesgelatinise at lower temperatures, giving a faster rate of gelatinisationwith a lower peak viscosity.

There are four major groups of dextrins: (i) hydrolysed starches withenzymes such as amylases; (ii) degraded products by acid hydrolysis;(iii) Schardinger dextrins, formed from dispersed starch by Bacillusmacerans transglycolase; (iv) Pyrodextrins, produced by the action ofheat alone or in combination with acid on dry, granular starch.

Products (i) and (ii) are generally prepared by the user at the site ofapplication. The Schardinger dextrins are of interest because of theirability to form inclusion complexes with organic molecules but are notpresently of major commercial interest.

Pyrodextrins are manufactured in large quantities and are of commercialinterest. Three types of chemical transformations take place during thedextrinization process. In the presence of acid and moisture,degradation by hydrolysis takes place. At high temperature and lowmoisture content, there is rearrangement involving breaking theglucoside linkage at one point in the molecule and reattachment at adifferent point. At high temperatures in the presence of acid andanhydrous conditions, the small fragments repolymerise to form highlybranched molecules. The conditions therefore of the dextrinizationdetermine the properties of the dextrin product. If hydrolysispredominates, then the dextrins will show retrogradation properties. Ifrepolymerisation and rearrangement predominates, then the dextrins willbe more stable to retrogradation and more soluble in cold water.

The pyrodextrinization process is carried out in four steps:acidification, predrying, dextrinization and cooling. Dextrinization(hydrolysis and repolymerization) is carried out until the products showthe desired characteristics of viscosity and solubility. Dextrinizatonis the roasting of dry starch, mostly in the presence of smallquantities of acid. Native starch (10-20% moisture) is mixed with therequired quantity of acid (usually hydrochloric acid). The next stage isa drying process to reduce the moisture content of the starch to levelsof 5-12% (white dextrins), or below 5% (yellow dextrins). The subsequentdextrinization process is carried out in rotating roasting kilns or influidised bed systems. When the reaction is complete the dextrin isdropped into a vessel and cooled. Finally the product is remoistened (toabout 10% moisture), sieved and bagged.

EXAMPLES

The effects of various starches on the properties of plasterboard wereinvestigated. In particular, the efficacy of the starches as corereinforcing agents was investigated. Nail pull resistance (according tothe standard ASTM C473-03 testing procedure) and compressive strengthwere measured. Additionally, the effect on paper-core bond strength wasmeasured. The starches used were:

-   -   A Native corn starch—RCO3408 (Cerestar Cargill)    -   B Acid thinned corn starch (control)—Collofilm 120 (Tate and        Lyle)    -   C Native potato starch—C*30002 (Cerestar Cargill)    -   D Waxy phosphate corn starch—C*06300 (Cerestar Cargill)    -   E Native waxy corn starch—C*04201 (Cerestar Cargill)    -   F High amylose starch—C*03003 (Cerestar Cargill)    -   G Thermally Modified Dextrin—C*Film 07311 (Cerestar Cargill)

Waxy phosphate corn starch and native waxy corn starch have a highamylopectin fraction. As the name suggests, high amylase starch has ahigh amylose fraction.

Compressive Strength

Prismatic gypsum blocks (with liner paper) were formed using slurryincluding the various starches tested as core strengthening agents. Foreach composition, 0.5% strengthening agent (starch) was added. Theresultant compressive strength of these blocks is set out in FIG. 1relative to the average compressive strength of the control samples(using acid thinned corn starch as the core strengthening agent).

Gypsum blocks measuring 20 mm in width, 20 mm in depth and 100 mm inlength, were produced for compressive test results. 70 ml of water per100 g of stucco (calcium sulphate hemihydrate) was mixed together in ahigh shear blender (Kenwood) for 10 seconds. The different grades ofstarches to be investigated were dry mixed with the stucco (calcium,sulphate hemihydrate) before producing the slurry. The slurry was pouredinto a rubber mould to fabricate six gypsum blocks measuring 20 mm×20mm×100 mm. The excess slurry was scraped off the top surface of themoulds with a spatula to produce a flat surface before allowing theslurry to set. The blocks were then removed from the moulds and placedin a custom made rig to hold 6 gypsum blocks together and dried in a180° C. oven with steam for 15 minutes. The blocks were then dried in a40° C. oven to constant weight before being conditioned at 23° C./50%relative humidity for a minimum of 24 hours. For starches C, D and F,additional blocks were made using a modified process, in which thegypsum blocks were dried in a 180° C. oven with steam for 18 minutes.Results for these are indicated in the drawings as C+, D+ and F+.

The gypsum blocks were tested in compression using a Zwick UniversalTesting Machine. Cubes of side 20 mm of the gypsum blocks were testedunder compression by placing a metal (20 mm side) cube on the surface ofthe gypsum block. A load at a constant rate of 250N/min+/−20% wasapplied and the peak load of failure measured and recorded. Each gradeof starch provided 12 compressive strength results.

Paper-Core Bond Strength

Plasterboard samples were manufactured in the laboratory, as follows.

A laboratory continuous mixer was used to produce the samples fortesting; it replicates the mixing regime of the board plant by having asimilar short residence time and dispersion.

The laboratory continuous mixer disperses the solid constituents (i.e.stucco and core strengthening agent) in the water and foam using a ‘IC’shaped rotor turning at high speed to form a plaster slurry that exitsfrom a spout mounted on the side of the rotor housing. Slurry iscollected in a mould on a plastic sheet with the bottom paper already inplace. When the mould is full, the top paper is placed on top; theexcess slurry is drawn out leveling the sample and another plastic sheetweighted down on top.

Slurry and resultant board properties were achieved by control of thefoam (sodium alcohol ether sulphate) and water flow rate considering thesolid constituents feed rate, which was controlled by the hopper linedwith vinyl internally to agitate the powder. To ensure the production ofa consistent slurry, feed rates for all components should be accuratelycontrolled and measured.

The starch was pre-blended into the bulk of the stucco before each runat the appropriate percentage addition level. GMN accelerator (GroundMineral Nansa, i.e. ground gypsum dihydrate with surfactant) was alsoadded at 0.3% by weight, based on the amount of stucco, to target afinal set time of less than 4 minutes. It is also possible to use HRA(Heat Resistant Accelerator) and BMA (Ball Milled Accelerator). Both ofthese are ground gypsum dihydrate with sugar to increase shelf life andpotency of the accelerator. When a dry additive was being assessed, atleast 5-6 kg was required to be dry blended for 20 minutes so theweighing system in the hopper could be balanced.

The following parameters were kept constant throughout:

-   -   Water gauge=70 ml/100 g    -   Stucco feed rate=60 kg/hr    -   Mixing water temperature=40° C.

A stainless steel lined experimental dryer having a temperature range ofAmbient-350° C., a constant humidity and high airflow was used to drythe samples in conditions similar to a typical plasterboardmanufacturing plant dryer.

The samples were allowed to hydrate at ambient temperature for 13minutes before being placed in the 180° C. drying oven for 15 minutes.(Again, some samples were heated at 180° C. in this step for 18 minutes,and a denoted by a + suffix.) The samples were then placed in a 60° C.oven for a minimum of 12 hours to remove any remaining moisture in theboard. The boards were then transferred to an oven set at 40° C. priorto being conditioned at 23° C./50% RH, re-weighed, thickness measuredand tested.

In each production run, the following samples were made:

150 mm×150 mm samples (×2) for nail pull testing according to the ASTMC473-03 Standard Test Methods for Physical Testing of Gypsum PanelProducts. The board thickness is 12.7 mm or ½ inch to emulate standardboard production on-line.

The amount of core strengthening agent added for each board type isshown in Tables 1 and 2.

The boards were also tested for paper-core bond strength. The bondstrength measurements were carried out at 20° and at 90% relativehumidity. The qualitative results are shown in Table 1. Whereappropriate, the results are shown for the compositions that weresubjected to a longer heating step of 18 minutes.

TABLE 1 Addition Additive level % Comments B (control) 0.5 Good papercore bond A 0.5 Good paper core bond C 0.5 Poor paper core bond F+ 0.5Average paper core bond E 0.5 Poor paper core bond D+ 0.5 Good papercore bond G 0.5 Good paper core bond

The numerical results are shown in the bar chart of FIG. 2.

Board samples were conditioned at 20 C and 90% relative humidity for 3hours. Using a utility knife, a figure ‘X’ was cut into the face side ofthe board at several locations across the width. The paper was peeledaway from the core. The percentage face bond was determined as the areacovered by paper plies. The procedure was repeated for the back side ofthe board. 100% bond would indicate no exposure of the gypsum core. Fullexposure of the gypsum core would indicate 0% bond.

Nail Pull Resistance

Nail pull resistance for each board was tested in accordance with theASTM standard. The results are shown in Table 2 and in FIG. 3.

TABLE 2 Starch Type % Addition level % Nail Pull B (control) 0.50 0 D1.75 Decreased 5% G 1.75 Increase 20% A 1.75 No change C 1.75 Decrease11% E 1.75 Decrease 4%

As shown in the scatter plot of FIG. 3, the nail pull resistance ofboards manufactured using dextrin as the core strengthening agent wassignificantly higher than that of the control boards, and significantlyhigher than that of the board manufactured with the same amounts ofalternative starches as potential core strengthening agents.

In Table 2, the % nail pull increase is calculated by taking the averagechange in nail pull strength for each class of samples with respect tothe best-fit line for the control samples (shown as a dotted line inFIG. 3).

Using the results shown in FIG. 3 and Table 2, dextrin was selected asthe most suitable strengthening agent to be tested in a full planttrial. Dextrin was added supplemental to acid thinned corn starch, andthe nail pull resistances of the standard board (using only acid thinnedcorn starch) and the dextrin boards were compared. The result was an 8%increase in nail pull for a board containing 0.5% dextrin and 0.5% acidthinned corn starch compared to 0.5% acid thinned corn starch alone(control). So, even at a supplementary amount of dextrin of only 0.5 wt% (based on the amount of calcined gypsum (stucco)), a significantincrease in nail pull resistance is seen.

From the selection of starches investigated, it was determined that thedextrinized starch was most beneficial to core strength of plasterboard.The dextrinized starch, which effected an improvement in the board'smechanical properties, was corn-based white dextrin starch. It enhancednail pull properties of plasterboard by up to 20% compared to standardplasterboard. At the addition levels of dextrin added in the process,0.5 wt % of stucco, no detrimental impact was seen. Good quality boardswere produced with good score and snap, and good mechanical properties,and enhanced nail pull resistance.

The level of dispersion throughout the core of the plasterboard can alsodetermine the performance of the starch. Uniform dispersion of thestarch results in enhancement of strength of the plasterboard. With theaid of iodine staining, it can be determined that dextrin that hasgelatinized and dispersed uniformly within the core provides an increasein nail pull resistance for the board. A darker stain represents agreater percentage of dextrin (or other starch) gelatinization. Theseresults are illustrated by FIG. 4, each sectioned plasterboard samplebeing labelled as previously.

In FIG. 4, the lightness of the iodine stained area of plasterboard isrepresented by the greyscale value of the image (0=black and 255=white)The greyscale values were determined by analysing the photograph shownin FIG. 4, in digital form on a PC using the software Paint Shop Pro 5.These values are shown in Table 3 below:

TABLE 3 Starch Grayscale Core Strength Nail Pull % type Value ChangeIncrease G  98 Good +20 A 105 ↓ No change B 105 (control) D 121  −5 E128  −4 C 158 Bad −11

FIG. 4 and Table 3 demonstrate that the darker the stain, the greaterthe nail pull strength enhancement seen. The lower the greyscale numberthe darker the stain, representing a fully gelatinised, uniformlydistributed starch within the core.

It is possible experimentally to determine the amount of dextrincontained in a particular region of the core. In a standard 12.7 mmboard, firstly the liner is removed. The remainder of the gypsum boardis sectioned into three parts, 2 mm, 8 mm and 2 mm, the cutting of thesections being parallel to the centre plane of the board. The 2 mmsections represent the interface of the gypsum panel and the 8 mmsection represents the central region of the core.

An enzymatic reaction can be used on the board samples to determine thetotal quantity of starch present in each of these samples. Anamyloglucosidase/α-amylase method allows the measurement of total starchwithin the sample. Starch hydrolysis proceeds in two phases. In phase I,the starch is hydrolysed and totally solubilised with the use ofα-amylase. In phase II, the soluble starch is quantitatively hydrolysedto glucose by amyloglucosidase.

For most samples, complete solubilisation of starch can be achieved bycooking the sample in the presence of thermostable α-amylase. However,for samples containing high levels of resistant starch (e.g. highamylose maize starch), complete solubilisation requires pre-treatmentwith dimethyl sulphoxide (DMSO) at 100° C.

After phase II is complete, samples containing high levels of glucoseand maltodextrins are then washed with aqueous ethanol before analysis.The analysis involves quantitative measurement of the levels of glucoseand maltodextrins present in the sample. An accuracy of +/−2% isachievable. This analysis is carried out according to the Megazymeamyloglucosidase/α-amylase method complying to the AOAC Method 996.11.

Preferred embodiments of the invention have been described by way ofexample. Modifications of these embodiments, further embodiments andmodifications thereof will be apparent to the skilled person on readingthis disclosure and as such are within the scope of the invention.

1. A method of forming a gypsum-based panel product including the stepsof: mixing calcined gypsum, water, a foaming agent and a corestrengthening agent to form a foamed slurry; depositing the slurry;shaping the slurry to form a board having a core and core surfaces,optionally sandwiched between sheets of liner; and allowing the core toset, and drying the board, wherein the strengthening agent is dextrin,used in an amount not less than 0.1% by weight and not more than 1.2% byweight, based on the amount of calcined gypsum, so that, after drying,there is provided a strengthening dispersion of dextrin throughout thecore.
 2. A method according to claim 1 wherein the strengthening agentis used in an amount not less than 0.3% by weight, based on the amountof calcined gypsum.
 3. A method according to claim 1, wherein theresultant panel has a nail pull resistance at least 4% greater than thatof a panel formed by the same method but without the strengtheningagent.
 4. A method according to claim 1, wherein the resultant panel hasa nail pull resistance at least 6% greater than that of a panel formedby the same method but without the strengthening agent.
 5. A methodaccording to claim 3, wherein the resultant panel has a core compressivestrength not less than that of a panel formed by the same method butwithout the strengthening agent.
 6. (canceled)
 7. A method according toclaim 1 wherein the dextrin is derived from one of corn, wheat, tapioca,potato, rice, sago, and sow gum.
 8. A method according to claim 1wherein the dextrin is added to the slurry in the form of granules.
 9. Amethod according to claim 1 wherein the dextrin has composition in whichthe amount of amylopectin is 70% or more, based on the total amount ofdextrin.
 10. A method according to claim 1 wherein 5% or less dextrin,by weight based on the amount of calcined gypsum, is added to theslurry.
 11. A method according to claim 1, wherein a liner is applied toat least one of the core surfaces and the resultant plasterboard has aliner-core bond strength not less than that of a plasterboard formed bythe same method but without the strengthening agent.
 12. A methodaccording to claim 11 wherein a paper-core bonding agent is added to theslurry, the paper-core bonding agent being different from thestrengthening agent.
 13. A method according to claim 12 wherein thepaper-core bonding agent migrates towards the paper-core interfaceduring processing of the plasterboard to a greater extent than the corestrengthening agent.
 14. A method according to claim 13 wherein thepaper-core bonding agent is an acid-modified or oxidised starch.
 15. Amethod according to claim 12 wherein the paper-core bonding agent isadded to the slurry in an amount not less than 0.1% by weight and notmore than 1% by weight, based on the amount of gypsum.
 16. A methodaccording to claim 15 wherein the paper-core bonding agent is added tothe slurry in an amount not more than 0.5% by weight, based on theamount of gypsum.
 17. A method according to claim 12 wherein the sum ofthe amount of paper-core bonding agent and core strengthening agent isless than 1.5% by weight, based on the amount of gypsum.
 18. A methodaccording to claim 1 wherein the panel product is produced in acontinuous process, the slurry being deposited onto a moving conveyor.19. A gypsum-based panel product having a core comprising at leastpartially hydrated calcined gypsum and a strengthening agent, the corehaving voids formed therein, the core optionally being sandwichedbetween sheets of liner, wherein the strengthening agent is dextrin,used in an amount not less than 0.1% by weight and not more than 1.2% byweight, based on the amount of calcined gypsum, the dextrin beingdistributed in the core so that there is provided a strengtheningdispersion of dextrin throughout the core.
 20. A gypsum-based panelproduct according to claim 19 comprising not less than 0.3% by weight ofstrengthening agent, based on the amount of calcined gypsum.
 21. Agypsum-based panel product according to claim 19 comprising 5% or lessdextrin, by weight based on the amount of calcined gypsum.
 22. Agypsum-based panel product according to claim 19 having a liner appliedto at least one of the core surfaces.
 23. A gypsum-based panel productaccording to claim 22 further comprising a paper-core bonding agent, thepaper-core bonding agent being different from the strengthening agent.24. A gypsum-based panel product according to claim 23 wherein thepaper-core bonding agent is an acid-modified or oxidised starch.
 25. Agypsum-based panel product according to claim 23 wherein the paper-corebonding agent is present in the plasterboard in an amount not less than0.1% by weight and not more than 1% by weight, based on the amount ofgypsum.
 26. A gypsum-based panel product according to claim 23 whereinsum of the amount of paper-core bonding agent and core strengtheningagent is less than 1.5% by weight, based on the amount of gypsum.
 27. Agypsum-based panel product according to claim 19 wherein the core has anotional centre line, equispaced between the surfaces of the panel, acentral region of the panel being defined as a band extending to twothirds of the thickness of the core and centred on the centre line,wherein, of the total amount of dextrin detectable in a cross section ofthe finished product, at least 50% is located in said central region ofthe panel.
 28. A gypsum-based board product according to claim 27including a paper-core bonding agent, wherein peripheral regions of thepanel are defined as: a first peripheral region of the plasterboardbeing defined in a band extending parallel to and incorporating oneboard surface and being 10% of the thickness of the board; a secondperipheral region of the plasterboard, at the opposite side of theplasterboard from the first peripheral region, said second peripheralregion being defined in a band extending parallel to and incorporatingthe other board surface and being 10% of the thickness of the board;wherein at least 50% of the total amount of paper-core bonding agentpresent in the board is located in the first and second peripheralregions of the board.
 29. A gypsum-based board product according toclaim 27 including a paper-core bonding agent, wherein at least 75% ofthe total amount of paper-core bonding agent present in the board islocated in the first and second peripheral regions of the board.